Various underfloor trench duct systems have heretofore been provided; however, because of certain inherent design characteristics, they have been beset with one or more of the following shortcomings: (a) the various components comprising the duct sections of the system were required to be custom-made at a predetermined manufacturing facility, thereby complicating the shipment of the components to a job site and often times causing inordinate delays in the installation of the system; (b) because the duct section components were custom-made, replacement thereof oftentimes was a serious and costly logistics problem; (c) modifying at the job site the length of either, or both, the cover plate and side rails of a duct section so as to compensate for unforeseen or unexpected structural situations or changes occurring during installation of the system was an extremely difficult, costly and laborious operation; (d) the coupler members or connectors utilized in interconnecting adjoining duct sections were frequently of costly construction, awkward to install and impaired the stability and continuity of the system; (e) the sub-assembly of the cover plate and side rails was not capable of being used in combination with either a base pan or being mounted directly onto a cellular sub-floor; and (f) warehousing of the various duct section components in numerous widespread geographical locations was not a practical or convenient way of distributing and merchandising the product.